The present invention relates to mufflers for use in various internal combustion engines including automobile engines, and more particularly to such an improved muffler which achieves highly enhanced sound deadening or muffling performance.
As examples of mufflers conventionally used in automobiles are known those as shown in FIGS. 13 and 14. The muffler 1 shown in FIG. 13 comprises four chambers 2, 3, 4 and 5 partitioned off from each other. Exhaust from an engine (not shown) first enters the chamber 4 by way of an entry pipe 6, then flows through a porous communication pipe 7 having a multiplicity of small through-holes into the chamber 2, from which the exhaust further flows through an exit pipe 8 to be discharged out of the muffler 1. The chamber 4 is in fluid communication with the chamber 5 via a pipe 9. The muffler 1 thus arranged can cause scattering or expansion, resonance, interference, resistance, etc. of the exhaust sound waves to effectively muffle the exhaust sounds in a predetermined frequency band.
The muffler 10 shown in FIG. 14 comprises a porous pipe 13 linearly connecting entry and exit pipes 11 and 12 for fluid communication therebetween, a filter 14 covering the outer periphery of the porous pipe 13, and sound absorbing material 15 filled between the filter 14 and the inner surface of the muffler chamber. In this example, sounds in a predetermined frequency band are muffled by virtue of the particular sound absorbing characteristic of the sound absorbing material 15.
The above-mentioned prior art muffler 1 of FIG. 13 is designed to reduce the sound pressure level by complication and prolongation of the exhaust flow path, thus achieving superior sound absorbing characteristics. However, this muffler 1 unavoidably suffers from increased loss and resistance of the exhaust pressure which would cause extremely adverse effects during high speed rotation under high load. The other prior art muffler 10 of FIG. 14 can exhaust linearly to thereby achieve a lower exhaust pressure loss, but it can only muffle sounds in a specific frequency band corresponding to the peculiar characteristic of the sound absorbing material and can never muffle sounds in low frequency ranges. In particular, no muffling can be achieved in a range of 70 to 90 Hz which is an excitation frequency of cavity resonance in vehicle chamber. In addition, the sound absorbing material easily absorbs moisture and hence tends to cause corrosion of the tubular exhaust paths and metallic components of the muffler body, resulting in reduced durability of the muffler.